In the field of implantable medical devices polymers based on L-lactic acid are quite favored due to the excellent biocompatibility of lactic acid. That is, lactic acid polymers biodegrade to smaller fragments and eventually to small molecules that are generally well-tolerated by the mammalian system.
When synthesizing poly(lactic acid) it is possible to use lactic acid itself as the monomer. The molecular weight of the polymer, however, is limited and if higher molecular weight product is desired, lactide, the dimer of lactic acid is the preferred monomer. For the purposes of this disclosure, poly(lactide) will be used to signify that the resultant polymers may have a broad range of molecular weights.
Constructs made of poly(lactide) exhibit good mechanical characteristics such as strength and tensile modulus. The fracture toughness of poly(lactide) is, however, lower than is often desired in a particular construct such as stents. The high strength and tensile modulus and concomitant low fracture toughness stems from the high degree of crystallinity of poly(lactide), which can be 55% or greater depending on polymer synthesis, construct-forming polymer formulation and construct fabrication parameters. In addition to the relatively high percent crystallinity, the crystalline structure of poly(lactic acid) in general comprises relatively large spherulites that add to the strength and tensile modulus of the polymer but detract from the fracture toughness.
What is needed is a lactide-based composition that can be fabricated into an implantable medical device that exhibits good strength, a good tensile modulus and good fracture toughness. The present invention provides such a composition and implantable medical devices fabricated of that composition.